how to extract dynamic array data from classes

I am trying to extract an array of data from a class. This is not all the code, but the "class num" takes in an array of data and now I want to extract this array data into my main function. But I am getting some weird numbers as output in the main function. (new to programming)

#include<iostream>
#include <string>
#include <fstream>
#include <sstream>
using namespace std;
class measure
{
public:
    virtual ~measure(){}
    virtual double*extract()=0;
};
class num:public measure
{
protected:    
    int rows;
    double *data;
public:
    num(){rows=0; data=0;}  //Intialise objects.
    ~num(){delete[] data;}
    double* extract();
};

So far for the extraction part, I have something like this:

double* num::extract()
{
    double *temp_arr;
    temp_arr = new double [rows];
    for (int j = 0; j <rows;j++)
    {
        temp_arr[j] = data[j];   
    }
return temp_arr;
}

And in the main, I have something like this:

num a;
double *b;
b = new double [rows];
b = a.extract();
for (int i= 0; i <rows; i++)
{
    cout<<b[i]<<endl;
}